Can more than one neutral or ground wire be terminated under the same lug/set-screw in an electric panel?
Sunday, July 29, 2018
Each neutral (white, grounded conductor) wire should be secured separately under its own lug/set-screw terminal in an electric panel, per National electrical Code (NEC 408.41). Also, a neutral and equipment ground (bare or green) wire cannot share a terminal. If you look carefully at the photo above, you will see that the panel has both defects, which is an indication that a handyman or homeowner has been in the panel adding circuits. Professional electricians know better.
The reason for the single wire per termination lug requirement is that placing multiple neutrals under one terminal makes it difficult to isolate a circuit for any troubleshooting. The branch circuit breaker is turned off when isolating a circuit, and the neutral is disconnected by removing it from the terminal. If there are two neutrals under the same terminal, the second neutral may also unintentionly be disconnected, even though the circuit is still energized, and this has the potential in some circuits to create an over-voltage condition in the circuit that is still “live.” Putting a neutral and ground under the same terminal creates a similar issue.
Although the NEC states that “each grounded conductor shall terminate within the panel board in an indiviual terminal that is not also used for another conductor,” it allows an exception for “grounded conductors of circuits with parallel conductors shall be permitted to terminate in a single terminal if the terminal is identified for connetion of more than one conductor."
A second exception to the “one wire per terminal” rule is that up to three ground wires can be secured at one terminal. This is likely because ground wires are intended for only temporary use and, if a ground wire becomes energized, a breaker should trip shortly afterwards.
Also, neutral and ground wires can be terminated along the same bus bar in a main service panel (the first panel after the meter), but they must be on separate bus bars in any subpanels, with neutral bus bar isolated (not bonded).
For more on this subject, see our blog post When did the code first set a limit of one neutral wire (grounded conductor) connection per lug/set-screw in an electric panel?
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